The present invention relates to a device for detecting a momentary cutoff or of an AC power source which is used with a data processing apparatus or the like.
Today, various kinds of data processing apparatuses are available and extensively used in various fields. A data processing apparatus is generally powered by an AC power source and constructed to prepare a DC power source from the AC power source so that various electronic circuits in the apparatus may be supplied with DC power from the DC power source. Assuming that the data processing apparatus is used for magnetically filing data, for example, data are accidentally and frequently destructed although the protection of data is a primary requisite. One of the causes of data destruction is the disturbance to the DC power source which results from a momentary cutoff of the AC power source. Momentary cutoff of an AC power source in turn is entailed by, for example, a failure of the power source itself or some critical accident occurred in another apparatus which shares the AC power source with the data processing apparatus. In any case, it is undesirable to deliver or write signals which have been effected by disturbance to the DC power source.
In order to protect data from disturbance to a DC power source which originates from a momentary cutoff of an AC power source, the momentary cutoff may be detected by comparing a DC reference voltage and a signal output from a capacitor-resistor (CR) intergrator circuit which is connected to the AC power source, as disclosed in Japanese Patent Laid-Open Publication No. 52-83144/1977. A problem encountered with this method is that, due to the use of a CR integrator circuit, it is incapable of responding to fast momentary cutoffs. Concerning the commercially available frequency (50 Hz or 60 Hz for example), the duration of cutoffs detectable with such a method is not shorter than about 10 milliseconds.